Abstract/Summary

This study considers fixed-wing airborne electromagnetic data obtained by a recent high-resolution airborne geophysical survey across the island of Anglesey, Wales. The survey detected an extensive (~10 km in length) highly conductive, near-surface zone across low-lying marsh and salt-marsh areas. Four-frequency electromagnetic survey data enable a continuous assessment of subsurface conductivity variations across the inter-tidal zone and onshore into Holocene sediments and bedrock. Inshore seawater conductivities exceed 2000 mS/m but reduce across sandbank areas to a range of values, typically exceeding 500 mS/m. Saline incursion onshore is observed largely at lower values and at a range of scales extending to over 10 km. Within defined salt-marsh areas, a main front of near-surface incursion can be mapped and extends several hundred metres inshore. The main body of subsurface saltwater incursion is largely confined to a trough containing Holocene deposits above Upper Carboniferous (coal) measures that were historically mined. The main conductive zone becomes more compact, both vertically and laterally, with increasing distance inland. Modelling of the data indicates that, onshore, the major concentrations of dissolved solids are confined to the upper 30 m within undulating subsurface zones. Historical tidal salt loading of the Holocene sediments may account for the main features detected within this low-lying and faultbounded area now protected by tidal gates.